Suppose a planetary system works in such a way that there is a rotating planet in the direct centre of a binary star (the star system's centre of gravity, also known as barycenter), such that the two stars appear to be orbiting around the stationary planet. If the stars are small, and the planet is an acceptable distance away, the temperature should be livable on the main planet.

What other physical issues (e.g. radiation, stellar winds) or social issues (e.g. religions) could arise from this planetary setup?

  • $\begingroup$ Just to be clear, the planet doesn't move (it's tidally locked)? $\endgroup$
    – Beta Decay
    Sep 17, 2014 at 5:58
  • $\begingroup$ @BetaDecay ​​​​​​​​​​​​​​​The planet doesn't move. (also, nice to see you here as well as PPCG). $\endgroup$
    – absinthe
    Sep 17, 2014 at 5:59
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    $\begingroup$ I'm not sure how physically stable a system with a planet locked in the middle of two stars is. Is that part of your question or do you mean physical issues as in "on the surface of the planet"? $\endgroup$
    – drat
    Sep 17, 2014 at 6:02
  • $\begingroup$ ​​​​​​​​​@drat Physical issues on the surface of the planet. Assume the setup of the stars and stuff works somehow. $\endgroup$
    – absinthe
    Sep 17, 2014 at 6:03
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    $\begingroup$ Isaac Asimov explores this in detail for a planet with five (I think) suns in his story Nightfall. Worth a read for inspiration and enjoyment. $\endgroup$
    – Linuxios
    Sep 30, 2014 at 15:09

5 Answers 5


The most obvious point is that there would be very little darkness on the surface of the world, at best you would get twilight twice a "day" unless there was a large axial tilt that would give a true day/night cycle at the poles. If the stars were sufficiently alike, they could well be indistinguishable from one-another, or if they are different, there could be periods of alternating colour i.e. blue days and red days, and/or bright days and dim days.

The world would have to be a sufficient distance away that radiated heat would pretty much equal radiant influx, so the suns would appear rather smaller than is the case with Earth and Sol. This would mean that in sunny weather, shadows would be hard-edged except at twilight, when there would be hardly any shadows at all, just dim light. With any sort of atmospheric scattering of light, the stars would never be seen except at the poles if there was sufficient axial tilt, so the sky would most likely vary only between blue and bright and pink/red and dull. Since the stars would not be visible from most of the world, this would place a more inward-oriented bias to the beliefs of sentient inhabitants.

Caves, being the only place that true darkness would ever be seen in most regions, are likely to be a source of wonder and terror to sentient surface-dwellers, and cave dwellers would hardly ever venture out into the sunlit areas where they could be seen. Life-forms would probably not evolve cyclic sleep-wake patterns, as dim periods would be relatively brief, so if any creature needed to sleep, it would have to do so in a safe place, guarded by wakeful friends or family, or be able to defend itself despite being asleep. It is likely that sleep would be something more like hibernation, if it occurred at all.

This is, of course, ignoring the fact that the barycenter of a binary pair of stars is an unstable place for a planet to be. It would be all to easy for the planet to slide out of the barycenter and fall into one or other of the stars, or at the very least end up in a close orbit around one where things would quickly get much, much hotter... At least, the world itself would not be able to have a moon, given the additional instability this would cause. However, the forces required to keep a world within the barycenter of a binary star system would be relatively small, so it is not impossible if there is some form of intervention, presumably of sentient origin.

Since light would be more-or-less omnipresent, I disagree with Beta Decays now-deleted answer that there would be one religion worshipping each star, unless the stars were very different in colour and/or size. A more likely focus of religion would be the darkness within caves (or the light outside them) - light-dwellers would be likely to have practically no night-vision at all, and cave dwellers would most likely not have very good sight and would be vulnerable to light-dwelling predators with good vision. From either direction, the opposite environment would be a mystery.



  • Different sleeping patterns: humans go in deep sleep primarily because we are not adapted to dark environments, and so we can use that time to restore energies etc. Other species have different sleep patterns; some species of sharks, for instance, alternate between "active" and "less active" periods: they are always aware, but they are less active and restore energy that way. In a world without darkness, it's possible that we would develop a less drastic different between awake state and resting state. Polyphasic sleep could be another alternative, I suggest you read Pavlina's experiment on the subject.
  • Different physical features: all this depends on the intensity of the light. Stronger irradiation from the sun could lead to darker skin tones, increased vascularization or hairless bodies to increase heat dissipation, smaller eyes, increased reuse of water inside the body if the planet is arid...planets are very complex, and life is possibly even worse in this aspect, a relatively small change can have radical consequences. Keep in mind I'm not an expert though, so maybe you'd like other, more knowledgeable people to expand on all these things.

  • Different physiology: a huge part of our biology is based on the day/night cycle: sunlight helps control our melatonin levels (which in turn influences our sleep), the absorption of vitamin D (with a variety of positive effects) and serotonin (makes you happier). Night and deep sleep too have positive effects on our bodies: our heart rate goes way down during the night, having way more active time could actually make heart problem rates rise, and you could imagine a whole spectrum of problems on this basis. I believe that if you're going for hard sci-fi as a standard, you should consider the importance of the day/night cycle and circadian rhythm to our biological makeup.


  • Duality: in a way, one could infer that the easiness with which thoughts about duality come to us is also influenced by the stark contrast between night and day (think about how we insinctively think something is either good or bad, or how we tend to neatly categorize things). I personally believe this to be a consequence of instincts and evolution (you need to know where everything stands to survive, lack of knowledge creates uneasiness), but the lack of darkness could have an impact on how things are seen: maybe there is no difference between the two suns, and so philosophers etc. are more prone to consider different things to be, on a fundamental level, equal or similar (they could express love and hate with the same word, for instance, recognising the strength and nature of the emotion as the same over the superficially different effects).
    Maybe, like another answer suggested, the suns could have different lights (red and blue), and the change could be very gradual (also depends on how quickly the stars are orbiting). This could encourage cultural and social fluidity, and a "grayer" way to view the world: maybe love and hate really are different, but it's very easy to go from one to the other, and not at all unexpected; maybe change in such a planet would be welcomed with little to no resistance, unlike our own world. Maybe positions of authority would change way more frequently, and opinion could be easily swayed (and no stigma would be attached to changing one's opinion frequently). There could also be less restrictive social norms (marriage and other long-standing traditions could possibly not exist. Maybe the whole concept of tradition would be alien to such a culture).
  • Cultural makeup: you have to think about the extent to which the suns could homogenize culture, and to this extent I would address you to questions relating to "planets of hats" (planets with a single cultural makeup). The basic idea is that for a culture to be very similar, you need similar circumstances: same basic terrain, same resources, same weather, same everything; if there are different circumstances, some form of different culture will naturally arise. And you also have to keep in mind the influence of government: if you have a single government (possible, but difficulty varies depending on the extension of the planet) either the problem doesn't exist already or the government would probably exert some sort of control over culture to keep the political situation manageable without splitting into local factions; if you hve multiple governments, well...why would they arise if the culture is homogenous and the needs of the various settlements are the same?
  • Religious: my understanding of religion-building is that you can really go crazy, as long as enough time has passed from the religion's foundation and said foundation is consistent with the world they're in. You could have two creator deities that become united in a single, supremely powerful being. Or, depending on the effects of the two suns on the land itself you could have two warring deities, or if for instance one sun is blue and the other is red, one of the deities could be killed every day and resurrect during the other's reign (which would explain the red sky, provided blood is still red). If the planet's inhabitants cannot see the suns they may come to worship the earth, or maybe there could be a sort of love triangle between the two suns and the earth. As long as you take the major "players" into consideration, the possibilities are limitless.


  • Progress: in the case of a fluid society, there would be no resistance to paradigm shifts in science, and progress could happen much more quickly.
  • Energy production: progress in solar power would be pretty much a given IF there is not a huge variability in wheather. If you're living in Space London, you can have all the suns in the universe, but solar power would still be a somewhat silly investment. You need reliable access to sunlight to turn a profit, energy-wise (at least you needed it five or six years ago, not sure about current level of technology).

Something to consider is the effect on weather and tidal patterns.

If the two suns are opposite each other then they will work together to create strong tides (by solar tide standards) but probably still not as strong as lunar tides.

Think about the spin of the planet compared to the movement of the two suns (since the suns will be orbiting each other).

With a vertical axis you have star A and star B with the planet spinning there.

   A    |    B

In this case you would have continuous daylight and warmth at the equator, continuous cooler twilight at the poles. Expect weather cycles with evaporation and warming happening at the equator then flowing towards the poles where it sinks and cools. At the equator you would have cycles that go noon A-twilight-noon B-twilight-noon A etc.

With sufficient water you would have tides moving around the planet on a one-day cycle.

If the axis of rotation moves away from the vertical then the suns would start to move in the sky in small circles over the course of a year. There might be some seasons although the differences would be smaller than we see.

One last special case though is if the planet is tidally locked. In this case the planet is spinning slowly enough that one complete turn of the planet is one year. This means that one point at the equator on each side you have permanent noon and extreme heating. As you move away from that point things would get cooler and darker, until eventually you started to see the second sun rising. The sun(s) would always be in the same apparent position(s) in the sky for any point on the planet.

This is actually more likely than it might seem in this already highly unusual star system as the gravity of the stars would work to pull the planet into that configuration. It's the same reason our moon always shows the same face towards us.

You would have no tides as the gravitational pull from the suns would not be moving with respect to the planet.


This is where things get really interesting. What would happen to life in this situation?

In the areas of constant sunlight then you could well get desertification, if there is enough water though then expect rampant plant-life benefiting from 24 hour photosynthesis. The constant light and warmth would be a benefit to cold blooded animals so expect them to compete more efficiently.

As you move away from the light conditions would change. Plants would need to cope with lower light levels and colder temperatures. Warm-blooded animals would take over from cold-blooded ones using their ability to regulate their body temperature.

Seasons and plant life

As already mentioned there wouldn't be any real seasons to speak of, even with axial tilt you would get modest variations. Because of this I'd expect plants to be evergreen. There would be no real concept of year or anual die-offs. You should consider how plants and animals would co-ordinate things like breeding cycles and pollination. Tropical ecosystems on earth would be a good source of inspiration here.

  • $\begingroup$ The A - B case does not take into account that stars orbit around the point where the planet is, so it is not correct. $\endgroup$
    – Envite
    Sep 17, 2014 at 22:08
  • $\begingroup$ @envite you are correct, it would need to be tidally locked instead - I don't have time this evening but I'll fix it tomorrow. $\endgroup$
    – Tim B
    Sep 17, 2014 at 22:10
  • $\begingroup$ @Envite Hopefully that's better :) $\endgroup$
    – Tim B
    Sep 18, 2014 at 8:06
  • $\begingroup$ @Tim_B It is better now, thanks for editing and glad to help. $\endgroup$
    – Envite
    Sep 18, 2014 at 8:25

There is a very interesting tale from Asimov about the consequences of perennial light: http://en.wikipedia.org/wiki/Nightfall_(Asimov_short_story)

There, there are several suns and always daytime, so no stars. Depending on the setup of your system you can have perennial daytime also, so you should check it and take the appropiate points.

Some of these points may be that a culture with no night could become unstoppably afraid of darkness, and consequently of closed spaces, to the point of a single periode of global darkness (an eclipse) causing the complete end of the Civilization.

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    $\begingroup$ I'm sorry to downvote but I don't think this answer adds a lot. It's effectively an example of where someone has done something similar. To improve this I'd give examples from the story (citing obviously) of the impacts it had on that civilisation. $\endgroup$
    – Liath
    Sep 18, 2014 at 7:53
  • $\begingroup$ @Liath I agree. think this would be a worthwhile comment but doesn't justify being an answer. $\endgroup$
    – Tim B
    Sep 18, 2014 at 8:07
  • $\begingroup$ I just wanted to avoid the complete argument, for the sake of those not having read it. Editing. $\endgroup$
    – Envite
    Sep 18, 2014 at 8:21
  • $\begingroup$ I thought of this book as soon as I read the question title, and think it would be a good read for the asker. Still, you could probably expand on this a bit to make it more of a complete answer. Maybe note how the people on the planet couldn't even conceive of the idea of night, how a simple dark tunnel with nothing else going on was a frightening theme park ride, that the idea of other planets was laughable nonsense, maybe mention the religion, so on. $\endgroup$
    – DCShannon
    Dec 21, 2015 at 18:17

Well, this question has been inactive for a long time, but it's interesting enough that I might try an answer anyhow.

I see a few different ways this could go, depending on the planet's rate of rotation.

  1. Planet is rotating at a speed comparable to Earth's. No axial tilt. In this case, the day proceeds in a cycle that looks like Earth's until sunset - whereupon, instead of twilight fading into darkness, the second sun picks up where the first one left off. The sunset-sunrise combo is presumably twice as romantic as anything we have on Earth. In this case, the suns reach their zenith at the equator, and the further north or south you travel, the less high they rise in the sky. At the poles, both suns hover at horizon-level and just go round and round without ever rising or setting, like two half-submerged luminescent nuclear sharks. It's cold there, like Earth's poles, and will probably feel creepy to someone from the more temperate regions, who's used to it being daylight most of the time. To sum up, this scenario is similar to Earth, except for the complete lack of either night time or seasons.
  2. Planet is rotating with respect to the suns, but slowly, like once per year. In this case, while we still have the daylight/twilight cycle, it becomes more of a seasonal thing: summer is noon, winter is twilight, morning and afternoon are spring and fall. Night time is still unknown, but prolonged periods of dimness now exist, even at the equators. These periods last long enough for it to get substantially cold, so expect to see Earth-like cycles of planting and harvesting crops and digging in for winter. At the poles, nothing much changes - the suns are still doing their peeking-over-the-horizon thing, they're just doing it much more slowly now.
  3. Throwing an axial tilt into the equation mixes things up a bit. Depending on the degrees of tilt and 'wobble', you could get:
    ~ Winter/summer cycles at the poles as they wobble seasonally towards one sun or the other, non-seasonal daylight/twilight cycles at the equator, and a mix of the two in between.
    ~ Permanent midafternoon (or any other time of day) at the poles, while the equators experience daylight/twilight (or winter/summer) cycles.
  4. This is my favorite scenario - because it's totally the best. If the planet is tidally locked, then we have:
    ~ One side always faces one sun, and the other side always faces the other.
    ~ No winter, no summer, no day, no night - the sun's position in the sky, and the temperature of the weather, don't vary with time. They vary only with your location on the surface of the planet.
    ~ On each side, there will be one exact spot where vertical objects cast no shadow. Expect to see a sacred pyramid at this location, surrounded by a major city. This is also the best region for growing crops, and for never being cold.
    ~ If the 'poles' of the planet are the warmest, brightest points directly beneath each sun, then the planet's 'equator' is the 'zone of twilight'. From this belt, and only here, both suns are visible - at all times. Caveat: they're so low in the sky that they provide little warmth, and this 'arctic equator' is always freezing. It's also (relatively) dark, and has flora and fauna drastically different from what can be seen nearer to the 'poles'. Thank penguins vs. parakeets, polar bears vs. tigers. Basically nothing which lives at the poles can survive at the 'equator', and vice versa.
    ~ From the perspective of a person living in one of the polar cities, traveling away from that city begins to look, at a certain point, like a journey into Hell. The sun sinks lower and lower in the sky, the temperature drops precipitously, the light grows dimmer and redder. Plants and animals become alien and hostile. The fact that these people have never known winter or night time, and are used to frolicking naked in the sun 24 hours a day, makes this particularly trying. Human habitation thins out and disappears. Any that do inhabit this wilderness are probably exiles and outlaws, and living a somewhat desperate existence.
    ~ Now imagine that they don't know that there's another sun on the other side. Their map of the known world then looks like this:enter image description here
    It could be hundreds or thousands of years before anyone is brave and tough enough to make it all the way through the icy hell and find this whole other world with exotic people, cultures, animals, plants, and perhaps a subtly different sunlight than their own.

Final note: some have said that this planet can't have a moon, because it would add too much instability. I disagree. I think it has to have one. As others have been quick to point out, the system is inherently unstable, because it's balanced on a gravitational pinpoint, and some artificial, responsive mechanism is needed to keep it from falling towards one or the other of the suns. Well, how is this mechanism going to be actuated? You can't strap rockets to the sides of the planet - you'll burn away the atmosphere. You can, however, put rockets (or ion engines, or whatever you like) on the moon. By subtly altering the orbit of the moon around the planet, it should be possible to nudge it this way and that through the effect of gravity, thus keeping it right in the center where it needs to be. This would also give the alien overlords who built the place a nice vantage point from which to observe their wacky experiment...


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